Messages in this thread | | | Date | Mon, 1 Nov 2021 13:27:54 +0100 | Subject | Re: [PATCH] serial: xilinx_uartps: Fix race condition causing stuck TX | From | Michal Simek <> |
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On 10/26/21 12:27, Anssi Hannula wrote: > xilinx_uartps .start_tx() clears TXEMPTY when enabling TXEMPTY to avoid > any previous TXEVENT event asserting the UART interrupt. This clear > operation is done immediately after filling the TX FIFO. > > However, if the bytes inserted by cdns_uart_handle_tx() are consumed by > the UART before the TXEMPTY is cleared, the clear operation eats the new > TXEMPTY event as well, causing cdns_uart_isr() to never receive the > TXEMPTY event. If there are bytes still queued in circbuf, TX will get > stuck as they will never get transferred to FIFO (unless new bytes are > queued to circbuf in which case .start_tx() is called again). > > While the racy missed TXEMPTY occurs fairly often with short data > sequences (e.g. write 1 byte), in those cases circbuf is usually empty > so no action on TXEMPTY would have been needed anyway. On the other > hand, longer data sequences make the race much more unlikely as UART > takes longer to consume the TX FIFO. Therefore it is rare for this race > to cause visible issues in general. > > Fix the race by clearing the TXEMPTY bit in ISR *before* filling the > FIFO. > > The TXEMPTY bit in ISR will only get asserted at the exact moment the > TX FIFO *becomes* empty, so clearing the bit before filling FIFO does > not cause an extra immediate assertion even if the FIFO is initially > empty. > > This is hard to reproduce directly on a normal system, but inserting > e.g. udelay(200) after cdns_uart_handle_tx(port), setting 4000000 baud, > and then running "dd if=/dev/zero bs=128 of=/dev/ttyPS0 count=50" > reliably reproduces the issue on my ZynqMP test system unless this fix > is applied. > > Fixes: 85baf542d54e ("tty: xuartps: support 64 byte FIFO size") > Signed-off-by: Anssi Hannula <anssi.hannula@bitwise.fi> > --- > drivers/tty/serial/xilinx_uartps.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/drivers/tty/serial/xilinx_uartps.c b/drivers/tty/serial/xilinx_uartps.c > index 962e522ccc45..d5e243908d9f 100644 > --- a/drivers/tty/serial/xilinx_uartps.c > +++ b/drivers/tty/serial/xilinx_uartps.c > @@ -601,9 +601,10 @@ static void cdns_uart_start_tx(struct uart_port *port) > if (uart_circ_empty(&port->state->xmit)) > return; > > + writel(CDNS_UART_IXR_TXEMPTY, port->membase + CDNS_UART_ISR); > + > cdns_uart_handle_tx(port); > > - writel(CDNS_UART_IXR_TXEMPTY, port->membase + CDNS_UART_ISR); > /* Enable the TX Empty interrupt */ > writel(CDNS_UART_IXR_TXEMPTY, port->membase + CDNS_UART_IER); > } >
Shubhrajyoti: Can you please take a look at this one?
Thanks, Michal
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